Method and apparatus for fabricating polymer-based electroluminescent displays

ABSTRACT

A method for fabricating an electroluminescent display and the substrate and apparatus used therein. The display is preferably constructed on a pre-constructed substrate that includes a flexible base layer having a conducting surface on one side thereof. The base layer is impermeable to oxygen and water. The substrate includes a plurality of wells defined by a barrier layer, each well having an electrode layer connected electrically with the conducting surface. A removable protective layer covering the wells protects the electrode layer from attack by oxygen and water prior to being utilized to make the display. In one embodiment, each of the wells also contains an electron transfer layer in contact with the electrode layer. The electron transfer layer includes a material that improves the efficiency of the injection of electrons from the electrode layer into the electroluminescent layer of the display. This display is fabricated by moving a first dispenser relative to the substrate so as to quantitatively deposit a first electroluminescent material on the substrate, different amounts of the first electroluminescent material being deposited at different locations on the substrate in response to signals defining an illumination pattern to be generated by the display. A conducting material is deposited on the first electroluminescent material. In color displays, a second electroluminescent material is also deposited, the second electroluminescent material emitting light at a different wavelength than the first electroluminescent material. A non-luminescent material, which is preferably an electrical insulator, can be deposited in wells at locations on the display that are not to emit light prior to depositing the conductive material.

FIELD OF THE INVENTION

The present invention relates to display devices, and more particularly,to displays utilizing polymer-based electroluminescent devices.

BACKGROUND OF THE INVENTION

Polymer-based electroluminescent devices (PLEDs) have the potential forproviding inexpensive alternatives to semiconductor-based LEDs. PLEDsmay be fabricated by coating the appropriate surfaces with an organicpolymer, and hence, do not require the use of high cost fabricationsystems such as those utilized in the fabrication of semiconductordevices. A simple PLED may be constructed from an electroluminescentlayer sandwiched between an electron injection electrode and a holeinjection electrode. More complicated devices utilize electron and holetransport layers between the above mentioned electrodes and theelectroluminescent layer.

In principle, PLEDs can be utilized to generate inexpensive displaysthat display a single fixed image of the type used in point of saleadvertising. Such a display would be self-illuminating, and hence, wouldreplace transparencies that are mounted on a light-box. If aconventional display is used, the image must be stored in a memoryexternal to the display and the display operated in a mode in which thestored image is continuously scanned into the display. The pixels of thedisplay must be individually addressable which further increases thecost of the display.

In spite of the lower construction costs inherent in PLEDs, multicolordisplays based on such devices are still quite costly to fabricate. Toconstruct a multicolor display, a patterned deposition of each of aplurality of electroluminescent compounds must be performed. In priorart fabrication systems, a series of masking operations is required toprotect the areas that are not to receive a particularelectroluminescent compound. The electroluminescent compound is thendeposited using vapor deposition, dipping, or spin casting. The mask isthen removed and the next mask constructed using conventionalphoto-resist techniques. Each masking operation increases thefabrication cost and reduces the device yield. In addition, theequipment and expertise needed to operate that equipment is of the typefound in semiconductor fabrication facilities. It would be advantageousto provide a system that could be operated by printers and the like.

Broadly, it is the object of the present invention to provide animproved display based on PLEDs and a method and apparatus forconstructing the same.

It is a further object of the present invention to provide a displaythat may be more inexpensively fabricated than prior art displays.

It is a still further object of the present invention to provide animproved method for fabricating PLED displays that does not requireexpertise in semiconductor fabricating techniques.

These and other objects of the present invention will become apparent tothose skilled in the art from the following detailed description of theinvention and the accompanying drawings.

SUMMARY OF THE INVENTION

The present invention is a method for fabricating an electroluminescentdisplay and the substrate and apparatus used therein. The display ispreferably constructed on a pre-constructed substrate that includes aflexible base layer having a conducting surface on one side thereof. Thebase layer is impermeable to oxygen and water. The substrate includes aplurality of wells defined by a barrier layer, each well having anelectrode layer connected electrically with the conducting surface. Aremovable protective layer covering the wells protects the conductivelayer from attack by oxygen and water prior to being utilized to makethe display. The electrode layer preferably includes a material chosenfrom a group of metals having a work function lower than 3.5 eV. Thismetal can be, for example, an alkali, alkaline earth or rare earthmetal. In some cases, instead of a low work function metal, a metallayer of Ag or Al can be used which is coated with a thin layer ofsuitable alkali or alkaline earth oxide or fluoride, such as CaO, Li₂O,MgO, LiF, MgF₂, CsF, or CaF₂. In one embodiment, each of the wells alsocontains an electron transport layer in contact with the electrodelayer. The electron transport layer includes a material that improvesthe efficiency of the injection of electrons from the electrode layerinto the electroluminescent layer of the display. The display isfabricated by moving a first dispenser relative to the substrate so asto quantitatively deposit a first electroluminescent material on thesubstrate, different amounts of the first electroluminescent materialbeing deposited at different locations on the substrate in response tosignals defining an illumination pattern to be generated by the display.A conducting material is deposited on the first electroluminescentmaterial. In color displays, a second electroluminescent material isalso deposited, the second electroluminescent material emitting light ata different wavelength than the first electroluminescent material. Inthe preferred embodiment of the present invention, the conductingmaterials include a hole transport material that facilitates theinjection of holes into the first electroluminescent material. Anon-luminescent material, which is preferably an electrical insulator,can be deposited in wells at locations on the display that are not toemit light prior to depositing the conductive material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a partially constructed displayaccording to the present invention.

FIG. 2 is a cross-sectional view of the starting material on which adisplay according to the present invention is preferably constructed.

FIG. 3 is a cross-sectional view of another embodiment of a displayaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The manner in which the present invention achieves its advantages may bemore easily understood with reference to FIG. 1, which illustrates thefabrication of a display 100 according to the present invention. FIG. 1is a cross-sectional view of display 100 part of the way through thefabrication process. Display 100 is constructed from a plurality ofpixels 110. Each pixel is a light-emitting element defined by a boundarywall 103 that contains the various materials that make up the pixel. Thevarious pixels are constructed on a flexible substrate 101 having aconducting layer 102 thereon. Layer 102 provides the electricalconnection to the bottom electrode in each pixel. Thesubstrate/electrode layer combination must be impermeable to water andoxygen to protect the cathode layer 104 that is deposited on top of theelectrode layer.

In the preferred embodiment of the present invention, the cathode layer104 associated with each pixel is deposited at the bottom of a welldefined by boundary wall 103. Cathode 104 is preferably constructed froma metal with a work function lower than 3.5 eV. This metal can be, forexample, an alkali, alkaline earth or rare earth metal. In some cases,instead of a low work function metal, a metal layer of Ag or Al can beused which is coated with a thin layer of suitable alkali or alkalineearth oxide or fluoride, such as CaO, Li₂O, MgO, LiF, MgF₂, CsF, orCaF₂. These cathode materials are highly reactive. Accordingly, thesubstrate/electrode layer must be impermeable to oxygen and water asdescribed above.

An electron transport layer 105 is then deposited on top of the cathodelayer. Electron transport layer 105 improves the efficiency of theinjection of electrons from cathode 104 into electroluminescent layer108 by providing a material that has an energy band between that of theelectroluminescent layer material and that of the cathode. In addition,electron transport layer 105 helps to protect the cathode material frommoisture and oxidization during the period of time in which thecathode/electron transport layer is so exposed for further depositions.

Refer now to FIG. 2, which is a cross-sectional view of the startingmaterial 125 on which a display according to the present invention ispreferably constructed. In the preferred embodiment of the presentinvention, the substrate and electrode layer together with the wellsdefined by the boundary walls are provided as a starting material withthe cathode and electron transport layers, 104 and 105, pre-deposited inthe wells. A protective oxygen/water impermeable sheet 121 covers thetop of the wells. This sheet is removed prior to placing the startingsheet in an apparatus that dispenses the electroluminescent layer 108and hole transport layer 107.

In general, a plurality of electroluminescent materials must be used forthe various pixels to provide a multicolor display. The ratio of thevarious components determines the color emitted by the pixels. The totalamount of material deposited determines the intensity of lightgenerated. The present invention makes use of the observation that thereare electroluminescent materials that are soluble in solvents that arecompatible with dispensing systems that can accurately dispense smallamounts of the solvents at precise locations. For example, the threeprimary colors may be generated from polyflourenes dissolved in anappropriate solvent with dopants to provide the colors. These compoundsmay be dissolved in xylene. The color is determined by the dye used inthe doping. Examples of suitable electroluminescent materials are knownto those skilled in the art. The reader is referred to“Electroluminescence of doped organic thin films”, Tang, et al., J.Applied Physics 55, pp 3610, 1989, which is hereby incorporated byreference, for a more detailed list of suitable materials. For example,the Coumarins and DCM compounds disclosed therein provide emissions inthe blue-green and orange-red portions of the spectrum, respectively.

To simplify the following discussion, a fabrication method based on adispensing system that is analogous to the mechanism used in inkjetprinters will be described. However, it will be obvious to those skilledin the art from the following discussion that other types of dispensersmay be utilized without departing from the teachings of the presentinvention. An inkjet printer dispenses small droplets by propelling thedroplets from a nozzle. In one system, the droplets are propelled byvaporizing part of the liquid and using the vaporized liquid to propelthe droplet toward the receiving surface. Resolutions of 1400 dropletsper inch are commonly achieved with inexpensive printer mechanisms.

Referring again to FIG. 1, each pixel location consists of a well whichis to receive one or more droplets of electroluminescent material. Thedroplets 138 are dispensed into the wells by a dispenser 136, whichmoves with respect to the display being fabricated on a rail 137. Asnoted above, dispenser 136 includes a plurality of reservoirs 139, onefor each electroluminescent material to be dispensed. In addition,dispenser 139 may include reservoirs for dispensing the materials usedto produce the hole transport layers discussed below as well as thefinal electrode. The electroluminescent materials are preferablyelectroluminescent dyes that are dissolved in a carrier liquid. In theembodiment shown in FIG. 1, three color reservoirs are utilized. As thereservoirs pass over the wells, controller 135 dispenses droplets intothe well in accordance with a predetermined pattern that is stored incontroller 135. In essence, controller 135 prints the pixel pattern onthe masked bottom electrode. Mask 103 prevents the droplets from runninginto one another.

After the droplets have dried, a hole transport layer (HTL) 107 isgenerated by dispensing an appropriate hole transport material into eachwell. A top electrode 109 is then deposited over the hole transportlayer to provide the electrical connections needed to power the device.The hole transport layer 107 may be constructed from a plurality oflayers having different conduction bands that more optimally match theenergy band of the electroluminescent layer (EL) to the top electrodematerial. In the preferred embodiment of the present invention, the topelectrode is transparent. For example, the top electrode may beconstructed from indium tin oxide (ITO). A second flexible substratelayer may be provided over the top electrode to further protect thedisplay from water/oxygen.

As noted above, the HTL layer can be constructed from one or morepolymer layers or other small molecule organics. For example, the HTLlayer may comprise any suitable organic conductor of holes, such astertiary amine or carbazole derivatives both in their small molecule ortheir polymer form, conducting polyaniline (Pani),polyethylenedioxythiophene-polystyrenesulfonate (PEDOT:PSS), thiophenes,metal-base or metal-free phthalocyanines, and many other suitablematerials that will be known to those of ordinary skill in the art.

An EL layer may be constructed from a polymer blend based onpolyfluorenes and PPV derivatives or other similar polymers as describedin U.S. Pat. Nos. 5,776,623, and 5,777,070 and 5,869,350. These polymersare soluble in xylenes, and are not soluble in water, acetone, orisopropyl alcohol. Hence, the EL layer can be deposited on the cathodemetal without introducing water.

In another case the electroluminescent layer may comprise small moleculematerials such as Alq₃ and other similar materials as described in U.S.Pat. Nos. 5,608,287 and 5,708,130, which are incorporated herein byreference.

In general, the final display will also have “dark” regions in additionto the various light-emitting pixels. If the hole transport layer orelectron transport layers also emit light, an electrical insulatingmaterial can be deposited in the pixels corresponding to the darkregions. The insulating material can be deposited in the same pass asthat used to deposit the electroluminescent materials into the otherwells. That is, a fourth dispenser can be provided to provide theinsulating material. In addition to preventing the dark areas fromgenerating light in the hole and electron transport layers, theinsulating material will lower the power consumption of the finaldisplay.

As noted above, the flexible substrate 101 preferably inhibits both theflow of oxygen and water. The substrate can be constructed from aflexible material such as poly (ethylene terephthalate) or PET, which iscommonly called polyester, poly (ethylene naphthalate), or transparentpolyimide. PET is used as a command substrate for Web processing.Unfortunately, PET has a water permeability that is so high that devicesconstructed thereon begin to degrade almost immediately due to thereaction of water from the air with the cathode material. Accordingly,some form of sealant coating must be applied to the polymer to achievethe required resistance to water and oxygen.

One such coating technique is the Polymer Multilayer (PML) techniquedescribed in U.S. Pat. Nos. 4,842,893, 4,954,371, and 5,260,095, whichare hereby incorporated by reference. In this technique, a coatingconsisting of a layer of polymer and a layer of an aluminum oxide isapplied to the flexible substrate to seal the substrate. Both thedeposition steps can be operated on Web processing equipment at veryhigh speeds. The resultant substrate has a resistance to water andoxygen permeation that is improved by three to four orders of magnituderelative to uncoated PET films.

While the above described embodiments utilized an inkjet printermechanism for dispensing the electroluminescent dyes, it will beapparent to those skilled in the art from the above discussion thatother dispensers may be utilized without departing from the teachings ofthe present invention. For example, dispensers based on vibratingnozzles are known to the art. In these devices the liquid leaving thevibrating nozzle breaks up into small droplets which are deflected by anelectrostatic field. Similarly, dispensers based on micro-pipettes areknown in the art. Other ink dispensers such as those used in penplotters may also be utilized.

Likewise, dispensers comprising an array of micro-pipettes forreplicating a pattern stored in a two dimensional array of wells areknown in the liquid handling arts utilized by biochemistry andmicrobiology. In this case, the pattern of pixels is stored in themaster array of wells and is replicated on each of the devices utilizingthe array of micro-pipettes. This technology is most applicable fordisplays having large pixels.

The above-described embodiments of the present invention utilized wallsto confine the droplets of electroluminescent materials. However, itwill be apparent to those skilled in the art from this discussion thatother forms of containment can be utilized. For example, the surface onwhich the droplets are deposited can be arranged in a plurality ofhydrophilic or hydrophobic regions so that the droplets are confined bysurface tension.

The above-described embodiments of the present invention have utilized“pixels” which are in the form of dots. However, other configurationscan be utilized. For example, display elements that are lines or morecomplex structures can also be utilized. Such complex elements can beused as sub-images or components of images.

The above-described embodiments of the present invention utilize adesign in which the cathode material is provided in the startingmaterial thereby eliminating the need for dispensing this material aspart of the fabrication process. These embodiments are preferred becausethe cathode material is highly reactive, and hence, must be depositedunder vacuum or in an inert atmosphere. If the printing processor hasvacuum sputtering equipment then embodiments of the present invention inwhich the anode material is deposited first can be practiced. Across-sectional view of a completed display 200 according to thisembodiment of the present invention is shown in FIG. 3. In such anembodiment, a layer of a transparent conducting oxide 202 such as ITO isdeposited on the flexible substrate 201. This anode layer can bedeposited in the bottom of the wells defined by barrier layer 203 asdescribed above or in place of the conducting layer under the boundarywalls as shown in FIG. 3. The hole transport material 204 is thendeposited on this layer. A prefabricated sheet with the flexiblesubstrate having the wells and anode can be provided as a startingmaterial in a manner analogous to that described above. The printingprocess then proceeds as discussed above by dispensing theelectroluminescent material 208 into each of the wells. After thedispensed material has dried, an electron transport material 207 isapplied over the top of each well. The sheet is then transferred to avacuum deposition apparatus and the cathode material 209 is applied overthe electron transport material layer. A protective coating 211 is thenapplied over the cathode material to protect the cathode material. Thevarious materials described above with respect to the embodimentsdiscussed with reference to FIGS. 1 and 2 may be utilized to implementthis embodiment of the present invention.

The above-identified embodiments have utilized hole transport and/orelectron transport layers. However, it will be obvious to those skilledin the art from the preceding discussion that embodiments in which theselayers are omitted may also be practiced if the energy bands of theelectroluminescent layer are positioned such that the efficiency ofholes and/or electrons from the relevant electrode into theelectroluminescent layer is acceptable.

Various modifications to the present invention will become apparent tothose skilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

What is claimed is:
 1. A substrate for forming a display devicecomprising an electroluminescent layer, said substrate comprising: aflexible base layer having a conducting surface on one side thereof,said base layer being impermeable to oxygen and water; a plurality ofwells defined by an electrically insulating barrier layer, each of saidwells having an electrode layer connected electrically with saidconducting surface; and a removable protective layer covering saidwells, said protective layer being impermeable to oxygen and water,wherein said electrode layer comprises a metal with a work functionlower than 3.5 eV.
 2. A substrate for forming a display devicecomprising an electroluminescent layer, said substrate comprising: aflexible base layer having a conducting surface on one side thereof,said base layer being impermeable to oxygen and water; a plurality ofwells defined by an electrically insulating barrier layer, each of saidwells having an electrode layer connected electrically with saidconducting surface; and a removable protective layer covering saidwells, said protective layer being impermeable to oxygen and water. 3.The substrate of claim 2, wherein said metal is an alkali, alkalineearth or rare earth metal.
 4. The substrate of claim 3, wherein saidmetal is chosen from the group consisting of Ca, Li, Al, Mg or an alloyof Ca, Li, Al, or Mg.
 5. The substrate of claim 2, wherein saidelectrode layer comprises a metal layer of Ag or Al with a layer of analkali or alkaline earth oxide or fluoride.
 6. The substrate of claim 5,wherein said alkali or alkaline earth oxide or fluoride comprises CaO,Li₂O, MgO, LiF, MgF₂, CsF, or CaF₂.
 7. The substrate of claim 2, whereinsaid electrode layer comprises ITO.
 8. The substrate of claim 7, furthercomprising a hole transport layer in contact with said electrode layer,said hole transport layer comprising a material that improves theefficiency of injection of holes from said electrode layer into saidelectroluminescent layer.
 9. The substrate of claim 2, wherein each ofsaid wells further comprises an electron transport layer in contact withsaid electrode layer, said electron transport layer comprising amaterial that improves the efficiency of injection of electrons fromsaid electrode layer into said electroluminescent layer.